Exploring garnet replacement for determining the durations of geological processes

• Applicant: Dr. Josephine Moore
• Subject Area: Mineralogy, Petrology and Geochemistry ; Geology
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 576516420

The metamorphic rock record preserves primary evidence for the conditions (pressure {P}, temperature {T}, composition {x}, and timing {t}) and rates of geological processes occurring during geodynamics processes such as orogenic cycles. This evidence is recorded by equilibrium mineral compositions and assemblages and by mineral zoning, produced by non-equilibrium processes. The fact that garnet can record both the equilibrium conditions (P, T, x, and t) and non-equilibrium processes in compositional zoning makes it an important metamorphic mineral. Correct interpretation of garnet compositional zoning, however, relies on the proper recognition of the mechanisms responsible for producing zonation. Whether grain boundary diffusion during epitaxial growth, post-growth modification by volume diffusion, or garnet-garnet replacement by dissolution-reprecipitation is responsible for the formation of zoning, each of these require a different interpretative framework. Our current incomplete characterization of dissolution-reprecipitation of garnet therefore represents a significant hurdle to the accurate interpretation of garnet compositional zoning. A better understanding of the compositional and a textural characteristics of dissolution-reprecipitation in garnet may have significant implications for the rates which we ascribe to metamorphic processes. As the rate of dissolution-reprecipitation is generally much faster than volume diffusion. Further, if gradual compositional profiles created during dissolution-reprecipitation of garnet are reproduced experimentally, then we can use the experimental constraints to make inferences on the conditions and rates of metamorphism in natural settings. The aim of this project is to describe and quantify the textural and compositional characteristics of dissolution-reprecipitation in garnet through a combination of experimental work, study of natural samples, and numerical reactive-transport modelling. The proposed project involves constraining the separate effects of T, t, and fluid composition on the extent and characteristics of garnet dissolution-reprecipitation reaction fronts through experimental work. Field based studies will be conducted so that experiments can then be compared and contrasted with garnet sampled from different metamorphic environments. The goal is then to use the constraints obtained from experiments, field-based studies and thermodynamic modelling to produce numerical reactive-transport models. Ultimately, these models will be used to provide more accurate constraints on the timescales of metamorphic processes.

• DFG Programme: Research Grants